In known electromedical implants, for example cardiac pacemakers, defibrillators, cardioverters or other electronically actuated or controlled implants which include a telemetry device for the communication of data to an external apparatus, the telemetry device has at least one transmitting device with a transmitter. Parts of the circuit for such a transmitting device, for example the HF-transmitting device, are usually in the form of integrated circuits, not least because of the ease of availability thereof.
Integrated circuits of that kind can admittedly be used in a versatile fashion and implement complex logic, but the variability thereof is only acquired, in particular in the high-frequency range, at the cost of a relatively level of energy consumption which is a matter of disadvantage in one of the stated implants, precisely in consideration of the severely restricted energy supply. In addition integrated circuits of that kind are also distinguished by a relatively high voltage requirement, which in turn also has a disadvantageous effect on the dimensioning of the energy supply.
Therefore the object of the present invention is to provide a UHF-transmitter for an electromedical implant, which has a lower level of energy consumption.
Based on an electromedical implant as set forth in the classifying portion of claim 1, that object is attained by the features recited in the characterizing portion of claim 1.
The invention includes the technical teaching that an electromedical implant with a lower level of energy consumption is obtained if discrete components which are optimized in terms of the purpose involved are used instead of integrated circuits. For that purpose in accordance with the invention the transmitter includes an oscillator which essentially comprises a first, preferably bipolar transistor and a resonator, and a second, preferably bipolar transistor which forms a buffer stage and an antenna driver. It is possible in that way in a simple fashion to afford a frequency-modulatable transmitter which can be optimally adapted for its use, with a low level of voltage and energy requirement.
A particularly stable frequency-modulatable transmitter can be achieved in advantageous variants in which the resonator is in the form of a SAW-resonator to form a SAW-stabilized oscillator.
In this case the transistors are preferably selected in consideration of maximum gain with the lowest collector current. In a further preferred feature the oscillator is in the form of a Colpitts or Clapp circuit. It is possible in that way to afford a particularly advantageous structure.
Preferred developments of the implant provide that a capacitance diode is arranged in series with the resonator, thereby permitting simple frequency modulation. Preferably, instead of the capacitance diode, for frequency modulation purposes, a PIN-diode switched by a third transistor is arranged in series with the resonator. In that way the frequency variation and thus the data rate and in addition or alternatively the range can be increased.
A simple wire loop or an open wire (throw-out antenna) in the contour of the header of the implant can serve as the antenna. Overall in this way for example at 400 MHz a current consumption of less than 1 mA is possible, at a range of several meters. The circuit can be supplied directly from a buffer capacitor fed by a high-resistance battery or from a low-resistance battery. A charge pump is advantageously not required.